Foamed packing materials have become a major component of the packing industry because of their strength, light weight, and shock absorptive and insulating capacities. Until recently nearly all of the foamed packing materials, loose and molded, were made from polystyrene resins. These resins have excellent properties, and form the standard against which other packing materials are measured.
Solid foamed packing materials are used wherever optimum inherent shock absorbance and thermal insulation are required. The former is the most important factor in solid molded packings for shipping protection of electronics and other fragile materials including business machines, electrical components, computers, tools, major appliances, hardware, and toys. The latter is more important in applications such as cups for hot liquids or molded packings for the insulation of warm foods. Cups for hot liquids obviously also depend upon non-dissolution of these materials in water.
The majority of plastics fall into the category of petro-plastics, which are a non-energy product of petroleum chemicals. Petroleum-based plastics are considered to be nonbiodegradable, or at best only slowly biodegradable. This, coupled with the amount of plastics produced and ending up as litter or in landfills, is primarily responsible for the activity towards plastics that are biodegradable. In the U.S. alone, about 58 billion pounds of petroleum-derived plastics were produced in 1989. Municipal solid waste contains 7% by weight and 17-25% by volume of plastics, largely from packing materials. While traditional plastics can be altered to enable facile chemical degradation, the toxicity of the residues have yet to be defined.
Replacement of petrochemically based plastics by biologically derived plastics would reduce petroleum usage. Litter from such plastics would disappear into its surroundings to leave only normal biological residues. Integrated waste management practices that include off-landfill composting of biodegradable wastes, incineration, some reduction of packaging materials, and recycling could help bring waste disposal under control.
For several years there has existed an interest in developing biodegradable loose packings from vegetable materials. These materials are generally made from corn and other starches and can include the addition of other materials which act to enhance polymerization, chemical crosslinking, or flexibility. These loose packings have been formed by a variety of standard foaming and extrusion methods derived from polystyrene foam production, or the extrusion or explosive popping of cereal foods. However, these largely starch-based materials are often not well suited for many applications of solid packing foams because of their relatively rapid breakdown under wet conditions, and their inherently low breaking strengths.